Try Guitar Rig 2.
It's awesome, in its fully awsome awsomeness.
Itworks as an AU as well as standalone.
There's a 15 days trial if you are really need to.
I usually practice using GarageBand's instruments, then master using GuitarRig2 / High quality
You may also want to add the Logic Denoiser at the beginning of your virtual stack in order to reduce the hiss (gaussian denoiser works ok for me, as I don't have a pream yet).
Rock out!